Low-melt viscosity, polymeric materials are generally preferred over higher melt viscosity materials because of their better processability during injection molding, melt extrusion, etc. This is particularly true for polymeric materials having relatively high melting points (Tm's), i.e., more than about 300.degree. C. Advantages realized by the use of such lower melt viscosity polymeric materials include (1) less energy consumption, (2) fewer safety and handling hazards, (3) less polymer degradation problems during processing, (4) faster mold filling in constrained geometries, (5) improved compounding with fillers, such as glass, talc, etc., and (6) greater ease to stir/pump/manufacture the material.
Essentially all thermoplastic resins are required to undergo flow in the molten state during the course of manufacture and fabrication into products. Thus, such important processing operations as extrusion, molding, calendering, etc., all involve the flow of molten, or relatively fluid, polymer. In such systems, the crystalline regions undergo melting in a fixed temperature range, and the polymer, above the melting temperature, behaves as a fluid rather than as a solid. As described herein, the melt viscosity of thermoplastic polymer systems is a measure of the flow characteristics of the material in the fluid or molten state under certain temperature and shear conditions. For liquid crystalline polymer systems above the temperature at which they become molten, a certain amount of order remains in the molten material which can lead to improved processing characteristics, especially low melt viscosity at high shear rates, such as during extrusion or injection molding. The melt viscosity of a thermoplastic polymer is affected by many variables, among which are such variables as temperature, rate of flow, polymer molecular weight, molecular weight distribution, branching, and chemical structure. It is generally agreed that lower melt viscosities are a distinct advantage during processing (extrusion, injection molding, etc.) of thermoplastic materials.
Another very important property of injection-molded polymeric materials is the tensile elongation (strain) at break. This is especially true for glass-filled, injection-molded liquid-crystalline polyesters that have at best relatively low tensile elongations (usually 2% or less). Since tensile strength and tensile elongation at break are very closely related to `toughness` of an injection-molded material and high `toughness` is highly desirable, it is very desirable to have the highest tensile elongation (strain) possible.
In addition to low melt viscosity and high tensile elongation, a relatively low polymer melting point is a very desirable property for thermoplastic materials, especially when the polymer melting point is greater than 300.degree. C. Lower melting points lead to lower processing temperatures that lead to better processability and less polymer degradation during processing.
Another desirable property for injection-molded, glass-filled LCP materials is a high heat distortion temperature, e.g., 265.degree. C. or higher. This is especially true for glass-filled, injection-molded liquid crystalline materials that are most often used in applications requiring resistance to very high temperatures, such as in circuit boards and electrical connectors that are often `soldered` by immersing in a molten solder bath.
Liquid crystalline polyesters (LCPs) are unique among polymers because they have very high tensile, flexural, and temperature resistance properties which are very desirable properties for high performance applications, such as in structural applications and in electronic applications. U.S. Pat. No. 4,169,933 discloses a group of liquid crystalline polyesters which consist essentially of residues of terephthalic acid, 2,6-naphthalenedicarboxylic acid, hydroquinone, and p-hydroxybenzoic acid.
There is a need in the art for LCPs to exhibit a very desirable and surprising combination of a relatively low melting point, a relatively low melt viscosity, a relatively high tensile elongation (strain) at break, and a relatively high heat distortion temperature. Properties that facilitate the injection molding of tougher, more heat resistant, intricate and thin cross-sectional designs, such as electrical connectors, used in computers and other electrical components are desirable. In fact, some of these intricate and thin cross-sectional designs are very difficult or impossible to injection mold using non-liquid crystalline polymers. This is especially true for mold designs containing numerous very small, thin-walled cavities and/or those mold designs that require the polymer to flow through relatively thin cross sections over a considerable distance. Therefore, LCP compositions that enable the molding of such relatively tough, complex/intricate designs are very useful.
It is, therefore, an object of this invention to provide a range of LCP compositions derived from terephthalic acid (T), 2,6-naphthalenedicarboxylic acid (N), hydroquinone (HQ), and p-hydroxybenzoic acid (PHB) that exhibit a very desirable combination of properties, i.e., a relatively low melt viscosity at typical processing temperatures, a relatively low melting point, and a relatively high tensile elongation (strain) and heat distortion temperature, when compounded with 30 glass fibers and injection molded.